This invention relates to a marine propulsion system, and more particularly to such a system employing counterrotating propellers.
It has been found that employing a single propeller in a marine propulsion system results in imbalanced propeller torque leading to hydrodynamic inefficiencies. For example, the torque load from the rotating propeller must be overcome during steering when pivoting the system about its substantially vertical steering axis. Utilization of dual counterrotating propellers substantially reduces or eliminates such inefficiencies.
The present invention discloses a marine propulsion system utilizing dual counterrotating propellers, and a drive system including shafts and gears for driving the propellers. Reference is made to copending application Ser. No. 07/242,357 filed Sept. 9, 1988 and entitled Chain Drive Marine Propulsion System With Dual Counterrotating Propellers. This application discloses a counterrotating propeller system, with the propellers driven by a chain driving mechanism. In accordance with the present invention, a dual counterrotating propeller system is driven by a gear and shaft arrangement, which facilitates a steerable gearcase construction. As will be explained, the drive system of the invention provides highly satisfactory steering through the steerable portion of the gearcase by substantially eliminating torque loads which result from employment of a single propeller driven by a shaft and gear system. First and second concentric propeller shafts are rotatably mounted in the lower portion of a depending gearcase. The propeller shafts are preferably coaxially mounted one within the other. A first propeller is mounted to the first propeller shaft, and a second propeller is mounted to the second propeller shaft. Rotatable shaft means extends longitudinally through the gearcase, and is interconnected with the engine output shaft so as to be rotatable in response to rotation thereof. Gear means is interposed between the shaft means and the first and second propeller shafts for providing counterrotation of the first and second propeller shafts, and thereby of the first and second propellers.
In one embodiment, the shaft means comprises inner and outer concentric and coaxial shafts, interconnected with the engine output shaft so as to be rotatable in response to rotation thereof. Counterrotation means is included in the system for providing counterrotation of the inner and outer shafts. The gear means preferably comprises a first driving gear connected to one of the longitudinally extending coaxial shafts, and second driving gear means connected to the other of such shafts. Each propeller shaft is preferably provided with a driven gear engageable with one of the driving gears. The driving and driven gears are arranged so that counterrotation of the inner and outer coaxial longitudinal shafts is transferred to the coaxial propeller shafts, to provide counterrotation of the propellers. With this construction, the counterrotation of the driving gears provides cancelling gear torque loads when steering the lower steerable portion of the gearcase about a steering axis defined by the axis of the inner and outer longitudinal shafts. Likewise, the counterrotation of the propellers also provides cancelling propeller torque loads, thus easing steering about the steering axis.
Satisfactory steering means is provided for effecting pivoting movement of the lower steerable portion of the gearcase about the steering axis. In one embodiment, the steering means may comprise a rack and pinion arrangement mounted to the upper portion of the gearcase and acting on a rotatable projecting portion of the lower steerable gearcase portion for pivoting the steerable portion of the gearcase about the steering axis.
The drive system of the invention is satisfactorily employed in either an outboard configuration or an inboard/outboard stern drive system, as will be explained.